The seasons keep more than one clock, and the first clock is Earth's orbit. Eccentricity measures how much the shape of Earth's orbit departs from a perfect circle, and it is the reason our seasons are slightly different lengths. Earth is closest to the Sun at perihelion, in early January, and farthest at aphelion, in July, as of November 2026. Astronomical seasons are based on the position of Earth in relation to the Sun, and the elliptical orbit makes them unequal. They vary in length between 89 and 93 days. Perihelion does not cause the seasons, but because Earth moves faster in early January, the seasons that occur during that time pass more quickly, and Earth travels about 2,000 miles per hour faster at perihelion than at aphelion. Kepler's second law explains why Earth speeds up near the Sun: the smaller the distance, the faster the motion. As a result, northern winter is about three days shorter than northern summer.
The distances are small. In 2023 Earth's center was 152,093,250 kilometers from the Sun's center at aphelion in July, and NASA gives the average as about 150 million kilometers. Perihelion increases the intensity of sunlight reaching Earth by about seven percent compared to aphelion, yet January is still northern winter, because Earth's axial tilt is the dominant driver of seasons.
The solstice is not the turning point of every measure. A National Weather Service office in the Quad Cities area of Iowa and Illinois says its earliest sunset of the year comes around December 8 and its latest sunrise near January 5, so those are not the winter solstice. Its earliest summer sunrise is near June 14 and its latest sunset near June 28. The hottest days of summer also come after the solstice, because land and oceans need extra weeks to warm up.
Weather services use another calendar. Meteorologists often use seasons that follow the months, and meteorological winter includes December, January and February. They use whole months because it becomes much easier to calculate seasonal statistics from the monthly statistics. These seasons are more even than astronomical ones, lasting about 90 to 92 days, from 90 days for winter of a non-leap year to 92 days for spring and summer.
The tropics keep a different timetable. Near the equator, the Intertropical Convergence Zone is a region where intense sun and warm water heat the air, raising its humidity and making it buoyant. Aided by converging trade winds, the air rises, expands, cools and releases its moisture in an almost perpetual series of thunderstorms. Seasonal shifts in the location of this zone drastically affect rainfall in many equatorial nations, and they produce the wet and dry seasons of the tropics. Higher latitudes have cold and warm seasons instead.
Plants and animals read these cycles. Phenology is the study of the timing and cyclical patterns of events in the natural world, and cues like day length, temperature and rainfall tell plants and animals when to act. Farmers and gardeners can use phenology to choose the best time to plant, fertilize and harvest crops. It also helps predict events such as allergies and mosquito season.
Living things add a clock of their own. The trigger that causes leaves to change color is the photoperiod, the length of the day and night. Shorter days signal trees to slow the production of chlorophyll. Cooling, but not freezing, autumn nights signal leaves to produce red, crimson and purple anthocyanins, which do not occur in leaves in spring and summer. They are thought to serve as a sort of sunscreen that protects leaves from bright sunlight. In a cool fall, red maple leaves appear brighter, and every tree has a different combination of pigments remaining. So the seasons run on more than one clock: the orbit, the calendar, the rain belt and the leaves.
Words to know
- eccentricity: orbit's stretch
- perihelion: closest to Sun
- photoperiod: day length
- anthocyanins: red pigments
- aphelion: farthest from Sun
